Where to put transmission fluid in a 1995 Subaru Legacy

This is not an electrical issue as you suspect.
These transmissions operate each gear you select by pressurizing the valve body passage that engages the gear. Get a transmission specialist to test/inspect the valve body to determine the actual cause. It could be as simple as original fluid in the car or the valve body is losing pressure. Worst case is the main pump is failing then getting a working used transmission would be the least expensive repair

lucas oil makes a good transmission leak product

Check for leaks, have those fixed if any. Check for wetness around the transmission pan. There are hoses in the front to the transmission cooler in the radiator. Make sure there's no leaks. Then replace the transmission filter and fluid. Make sure to add Lucas Transmission Fix at least this one time. I vouch for it, its not a miracle in a bottle. If you have no mechanical or electrical problems with it. It will shift better that new. It is important that you add fluid in increments as to not over fill the transmission. If you do, take some out immediately. If not it will find its own way out. You can add the fluid while the vehicle is on, running in park. Just don't get injured, fast rotating parts.

First off, if it's an automatic transmission you DON'T put 90 wt oil in it! It uses automatic transmission fluid. Depending how low it's down it could take anywhere from 3 - 6 qts of automatic trans fluid. Have a shop look at it for you.

The dipstick has a black plastic handle in the shape of ring of about 1 inch in diameter. It is located on the left side of the engine (looking from front) behind the spark plugs. Best visible from the passenger side fender location (start looking from the left part of the transmission , the black handle clearly stands out on the white metal).

First Did you take the emergency barke off? and have you checked the fluid level? even a pint of fluid can damage transmissions. look at fluid is it brown? if it is you need to change the oil and filter in the transmisson. Does it go in reverse?

if by a/t you mean automatic transmission try a power purge transmission service it may have a filter sensor that is telling you to service soon

automatic trans 2.2liter engine- 1990-1993/ 8.8 qts.
1994-on/ 8.4 qts
2.5 liter engines 10 qts. all years

manual trans 2.2 liter 2wd 1990-1994 3.5 qts
1995 2wd 3.7 qts

4wd 1990-1994 4.2 qts
4wd 1995 on 3.7 qts2.5 liter aall years 3.7 qts.

The Dipstick is on the driver side right next to the starter and yes you also put the fluid in there.

is located in the passanger side of the engine. almost right above the universal joint that powers the tires/ it's on the BELL HOUSING if you open the hood and look close you will see a 2 inch curved handle/ it's verry small to see but if you take your time you will see it. the dipstic itself its about 4 inches long . you have to use a funnel and pour the fluid inthe dipstick hole.